X-ray Tomographic Inspection System for the Identification of Specific Target Items

ABSTRACT

The present invention provides for an improved scanning process with a stationary X-ray source arranged to generate X-rays from a plurality of X-ray source positions around a scanning region, a first set of detectors arranged to detect X-rays transmitted through the scanning region, and at least one processor arranged to process outputs from the first set of detectors to generate tomographic image data. The X-ray screening system is used in combination with other screening technologies, such as NQR-based screening, X-ray diffraction based screening, X-ray back-scatter based screening, or Trace Detection based screening.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/870,407, filed on Apr. 25, 2013, which is a continuation ofU.S. patent application Ser. No. 12/788,083, filed on May 26, 2010, nowU.S. Pat. No. 8,451,974, issued on May 28, 2013, which relies on U.S.Provisional Patent Application No. 61/181,070 filed on May 26, 2009, forpriority.

The '083 application is a continuation-in-part of U.S. patentapplication Ser. No. 12/485,897, filed on Jun. 16, 2009, which is acontinuation of U.S. patent application Ser. No. 10/554,656, filed onOct. 25, 2005, and now issued U.S. Pat. No. 7,564,939, which is a 371national stage application of PCT/GB04/01729, filed on Apr. 23, 2004 andwhich, in turn, relies on Great Britain Application No. 0309387.9, filedon Apr. 25, 2003, for priority.

The '083 application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/371,853, filed on Feb. 16, 2009, which is acontinuation of U.S. patent application Ser. No. 10/554,975, filed onOct. 25, 2005, and now issued U.S. Pat. No. 7,512,215, which is a 371national stage application of PCT/GB2004/01741, filed on Apr. 23, 2004and which, in turn, relies on Great Britain Application Number0309383.8, filed on Apr. 25, 2003, for priority.

The '083 application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/651,479, filed on Jan. 3, 2010, which is acontinuation of U.S. patent application Ser. No. 10/554,654, filed onOct. 25, 2005, and now issued U.S. Pat. No. 7,664,230, which is a 371national stage application of PCT/GB2004/001731, filed on Apr. 23, 2004and which, in turn, relies on Great Britain Patent Application Number0309371.3, filed on Apr. 25, 2003, for priority.

The '083 application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/364,067, filed on Feb. 2, 2009, which is acontinuation of U.S. patent application Ser. No. 12/033,035, filed onFeb. 19, 2008, and now issued U.S. Pat. No. 7,505,563, which is acontinuation of U.S. patent application Ser. No. 10/554,569, filed onOct. 25, 2005, and now issued U.S. Pat. No. 7,349,525, which is a 371national stage filing of PCT/GB04/001732, filed on Apr. 23, 2004 andwhich, in turn, relies on Great Britain Patent Application Number0309374.7, filed on Apr. 25, 2003, for priority.

The '083 application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/758,764, filed on Apr. 12, 2010, which is acontinuation of U.S. patent application Ser. No. 12/211,219, filed onSep. 16, 2008, and now issued U.S. Pat. No. 7,724,868, which is acontinuation of U.S. patent Ser. No. 10/554,655, filed on Oct. 25, 2005,and now issued U.S. Pat. No. 7,440,543, which is a 371 national stageapplication of PCT/GB2004/001751, filed on Apr. 23, 2004, and which, inturn, relies on Great Britain Patent Application Number 0309385.3, filedon Apr. 25, 2003, for priority.

The '083 application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/697,073, filed on Jan. 29, 2010, which is acontinuation of U.S. patent application Ser. No. 10/554,570, filed onOct. 25, 2005, and now issued U.S. Pat. No. 7,684,538, which is a 371national stage application of PCT/GB2004/001747, filed on Apr. 23, 2004,and which, in turn, relies on Great Britain Patent Application Number0309379.6, filed on Apr. 25, 2003, for priority.

The '083 application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/097,422, filed on Jun. 13, 2008, and U.S. patentapplication Ser. No. 12/142,005, filed on Jun. 19, 2008, both of whichare 371 national stage applications of PCT/GB2006/004684, filed on Dec.15, 2006, which, in turn, relies on Great Britain Patent ApplicationNumber 0525593.0, filed on Dec. 16, 2005, for priority.

The '083 application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/478,757, filed on Jun. 4, 2009, which is acontinuation of U.S. patent application Ser. No. 12/364,067, filed onFeb. 2, 2009, which is a continuation of U.S. patent application Ser.No. 12/033,035, filed on Feb. 19, 2008, and now issued U.S. Pat. No.7,505,563, which is a continuation of U.S. patent application Ser. No.10/554,569, filed on Oct. 25, 2005, and now issued U.S. Pat. No.7,349,525, which is a 371 national stage filing of PCT/GB04/001732,filed on Apr. 23, 2004 and which, in turn, relies on Great BritainPatent Application Number 0309374.7, filed on Apr. 25, 2003, forpriority. In addition, U.S. patent application number relies on GreatBritain Patent Application Number 0812864.7, filed on Jul. 15, 2008, forpriority.

The '083 application is also a continuation-in part of U.S. patentapplication Ser. No. 12/712,476, filed on Feb. 25, 2010, which relies onU.S. Provisional Patent Application No. 61/155,572 filed on Feb. 26,2009 and Great Britain Patent Application No. 0903198.0 filed on Feb.25, 2009, for priority.

The '083 application is also a continuation-in-part of U.S. patentapplication Ser. No. 12/505,659, filed on Jul. 20, 2009, which is acontinuation of U.S. patent application Ser. No. 12/175,599, filed onJul. 18, 2008, which, in turn, is a continuation of U.S. patentapplication Ser. No. 10/952,665, filed on Sep. 29, 2004.

Each of the aforementioned PCT, foreign, and U.S. applications, and anyapplications related thereto, is herein incorporated by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates to X-ray scanning and, particularly to thesecurity screening of baggage and packages for contraband and suspiciousobjects, such as sharp objects, knives, nuclear materials, tobacco,currency, narcotics, and liquids.

BACKGROUND OF THE INVENTION

X-ray computed tomography (CT) scanners have been used in securityscreening in airports for several years. A conventional system comprisesan X-ray tube that is rotated about an axis with an arcuate X-raydetector that is rotated at the same speed around the same axis. Theconveyor belt on which the baggage is carried is placed within asuitable aperture around the central axis of rotation, and moved alongthe axis as the tube is rotated. A fan-beam of X-radiation passes fromthe source through the object to be inspected to the X-ray detectorarray.

The X-ray detector array records the intensity of X-rays passed throughthe object to be inspected at several locations along its length. Oneset of projection data is recorded at each of a number of source angles.From these recorded X-ray intensities, it is possible to form atomographic (cross-sectional) image, typically by means of a filteredback projection algorithm. In order to produce an accurate tomographicimage of an object, such as a bag or package, it can be shown that thereis a requirement that the X-ray source pass through every plane throughthe object. In the arrangement described above, this is achieved by therotational scanning of the X-ray source, and the longitudinal motion ofthe conveyor on which the object is carried.

In this type of system the rate at which X-ray tomographic scans can becollected is dependent on the speed of rotation of the gantry that holdsthe X-ray source and detector array. In a modern CT gantry, the entiretube-detector assembly and gantry will complete two to four revolutionsper second. This allows up to four or eight tomographic scans to becollected per second respectively.

As the state-of-the-art has developed, the single ring of X-raydetectors has been replaced by multiple rings of detectors. This allowsmany slices (typically 8) to be scanned simultaneously and reconstructedusing filtered back projection methods adapted from the single scanmachines. With a continuous movement of the conveyor through the imagingsystem, the source describes a helical scanning motion about the object.This allows a more sophisticated cone-beam image reconstruction methodto be applied that can in principle offer a more accurate volume imagereconstruction.

In a further development, swept electron beam scanners have beendemonstrated in medical applications whereby the mechanical scanningmotion of the X-ray source and detectors is eliminated, being replacedby a continuous ring (or rings) of X-ray detectors that surround theobject under inspection with a moving X-ray source being generated as aresult of sweeping an electron beam around an arcuate anode. This allowsimages to be obtained more rapidly than in conventional scanners.However, because the electron source lies on the axis of rotation, suchswept beam scanners are not compatible with conveyor systems whichthemselves pass close, and parallel, to the axis of rotation.

The present invention provides an X-ray scanning system for inspectingitems, the system comprising an X-ray source extending around a scanningvolume, and defining a plurality of source points from which X-rays canbe directed through the scanning volume, an X-ray detector array alsoextending around the scanning volume and arranged to detect X-rays fromthe source points which have passed through the scanning volume andproduce output signals dependent on the detected X-rays, and a conveyorarranged to convey the items through the scanning volume.

The present invention further provides a networked inspection systemcomprising an X-ray scanning system, a workstation and connection meansarranged to connect the scanning system to the workstation, the scanningsystem comprising an X-ray source extending around a scanning volume,and defining a plurality of source points from which X-rays can bedirected through the scanning volume, an X-ray detector array alsoextending around the scanning volume and arranged to detect X-rays fromthe source points which have passed through the scanning volume andproduce output signals dependent on the detected X-rays, and a conveyorarranged to convey the items through the scanning volume.

The present invention further provides a sorting system for sortingitems, the system comprising a tomographic scanner arranged to scan aplurality of scanning regions of each item thereby to produce a scanneroutput, analyzing means arranged to analyze the scanner output andallocate each item to one of a plurality of categories at least partlyon the basis of the scanner output, and sorting means arranged to sortitems at least partly on the basis of the categories to which they havebeen allocated.

The present invention further provides an X-ray scanning systemcomprising an X-ray source arranged to generate X-rays from a pluralityof X-ray source positions around a scanning region, a first set ofdetectors arranged to detect X-rays transmitted through the scanningregion, a second set of detectors arranged to detect X-rays scatteredwithin the scanning region, and processing means arranged to processoutputs from the first set of detectors to generate image data whichdefines an image of the scanning region, to analyze the image data toidentify an object within the image, and to process the outputs from thesecond set of detectors to generate scattering data, and to associateparts of the scattering data with the object.

The present invention further provides a data collecting system forcollecting data from an X-ray scanner, the system comprising a memoryhaving a plurality of areas each being associated with a respective areaof an image, data input means arranged to receive input data from aplurality of X-ray detectors in a predetermined sequence, processingmeans arranged to generate from the input data X-ray transmission dataand X-ray scattering data associated with each of the areas of theimage, and to store the X-ray transmission data and the X-ray scatteringdata in the appropriate memory areas.

The present invention further provides an X-ray scanning systemcomprising a scanner arranged to scan an object to generate scanningdata defining a tomographic X-ray image of the object, and processingmeans arranged to analyze the scanning data to extract at least oneparameter of the image data and to allocate the object to one of aplurality of categories on the basis of the at least one parameter.

Furthermore, there exists a requirement to screen baggage and cargoitems for the presence of explosive materials and explosive devices.Such a scan typically must be performed at a high speed, as measured inbaggage and cargo item throughput, but with a high standard of detectionperformance and a reduced false alarm level. False alarms that aregenerated require further inspection, which may involve reconciliationof the baggage or cargo item with the owner of that item prior to amanual search. Such processes are expensive and time consuming.

There is also a need to combine a high throughput tomography system witha secondary system capable of specifically detecting explosive devices.One or more two-dimensional X-ray images are acquired at one or morevarious projection angles at high speed (typically with a conveyor speedof 0.5 m/s). An automated algorithm analyzes these images for thepresence of a likely threat material or device. In the event that such adevice or material is found, the item of baggage of cargo is routed to asecond system which can form one or more tomographic slicereconstructions through the item. Due to the slow speed of knownsystems, only a small fraction of baggage and cargo items can bescreened in this way. The tomographic image or images is/are thenanalyzed by an automated explosives detection algorithm. Frequently, thealgorithm will raise an alarm on the baggage or cargo item and the imagedata must then be viewed by a human operator. The fraction of items thatcontinue to raise an alarm at this point are then subject toreconciliation and human search.

SUMMARY OF THE INVENTION

The present invention is directed toward a system for identifyingobjects in a container, such as cargo or luggage, comprising a firstscreening system and second screening system. The first screening systemcomprises a stationary X-ray source arranged to generate X-rays from aplurality of X-ray source positions around a scanning region; a firstset of detectors arranged to detect X-rays transmitted through thescanning region; a second set of detectors arranged to detect X-raysscattered within the scanning region; at least one processor configuredto process data output from the first set of detectors and generate atleast one tomographic image and to process data output from the secondset of detectors to generate scatter image data; and a second screeningsystem comprising at least one of a NQR-based screening system, X-raydiffraction based screening system, X-ray back-scatter based screeningsystem, or trace detection based screening system.

Optionally, the at least one processor outputs data indicative of asuspect object in the container. A suspect object is any object that isthe subject of interest by persons operating the X-ray scanning system,such as threat objects, illegal objects, contraband, weapons, narcotics,nuclear materials, currency, tobacco, knives, bombs, and other suchitems. The at least one processor outputs a signal indicating saidcontainer should be subject to the second screening system only if thefirst screening system identifies a suspect object in the container. Theat least one processor outputs a signal indicating said container shouldnot be subject to the second screening system only if the firstscreening system does not identify a suspect object in the container.The second screening system outputs a signal indicative of whether asuspect object exists in the container and said output of the secondscreening system, said tomographic image data, and said scatter imagedata are used to determine if the suspect object is illegal.

The first screening system operates in parallel with said secondscreening system. The first screening system operates serially withrespect to said second screening system. The first screening systemanalyzes at least one of the tomographic image data or scatter imagedata to determine a type of material of an object in the enclosure. Thesecond screening system conducts a nuclear quadrupole measurement basedon the type of material determined by the first screening system. Thesecond screening system conducts an X-ray diffraction based screeningbased on the tomographic image generated by the first screening system.The stationary X-ray source is an electronically scanned X-ray source.

In another embodiment, the present invention comprises a system foridentifying objects in a container that comprises a first screeningsystem and a conveyor system that moves a container to a secondscreening system. The conveyor can be any motive mechanism, including aconventional conveyor belt, carts, manually manipulated pallets, lifts,or other structures. The first screening system comprises a stationaryX-ray source arranged to generate X-rays from a plurality of X-raysource positions around a scanning region; a first set of detectorsarranged to detect X-rays transmitted through the scanning region; atleast one processor configured to process data output from the first setof detectors and generate at least one tomographic image; and aconveying system to move said container from the first screening systemto a second screening system, wherein the second screening systemcomprises at least one of a NQR-based screening system, X-raydiffraction based screening system, X-ray back-scatter based screeningsystem, or trace detection based screening system.

Optionally, the at least one processor outputs data indicative of asuspect object in the container. The at least one processor outputs asignal indicating said container should be subject to the secondscreening system only if the first screening system identifies a suspectobject in the container. The second screening system outputs a signalindicative of whether a suspect object exists in the container and saidoutput of the second screening system and said tomographic image dataare used to determine if the suspect object is a threat. The firstscreening system operates in parallel with said second screening system.The first screening system operates serially with respect to said secondscreening system. The first screening system analyzes the tomographicimage data to determine a type of material of an object in theenclosure. The second screening system conducts a nuclear quadrupolemeasurement based on the type of material determined by the firstscreening system. The second screening system conducts an X-raydiffraction based screening based on the tomographic image generated bythe first screening system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings in which:

FIG. 1 is a longitudinal section of a real time tomography securityscanning system according to a first embodiment of the invention;

FIG. 1a is a perspective view of an X-ray source of the system of FIG.1;

FIG. 2 is a plan view of the system of FIG. 1;

FIG. 3 is a schematic side view of the system of FIG. 1;

FIG. 4 is a schematic diagram of a data acquisition system forming partof the system of FIG. 1;

FIG. 5 is a schematic diagram of a threat detection system forming partof the system of FIG. 1;

FIG. 6 is a schematic diagram of a baggage sorting system according toan embodiment of the invention including the scanning system of FIG. 1;

FIG. 7 is a schematic diagram of a baggage sorting system according to afurther embodiment of the invention;

FIG. 8a is a schematic diagram of baggage sorting systems according tofurther embodiments of the invention;

FIG. 8b is another schematic diagram of baggage sorting systemsaccording to further embodiments of the invention;

FIG. 8c is another schematic diagram of baggage sorting systemsaccording to further embodiments of the invention;

FIG. 9 is a schematic diagram of a networked baggage sorting systemaccording to a further embodiment of the invention;

FIG. 10 is a schematic plan view of a stand-alone scanning systemaccording to a further embodiment of the invention;

FIG. 11 is a schematic side view of the system of FIG. 10;

FIG. 12 is a schematic side view of a modular scanning system accordingto a further embodiment of the invention;

FIG. 13 is a diagram of an X-ray scattering event;

FIG. 14 is a longitudinal section through a security scanning systemaccording to a further embodiment of the invention;

FIG. 15 is a further longitudinal section through the system of FIG. 14showing how different scatter events are detected;

FIG. 16 is a transverse section through the system of FIG. 14;

FIG. 17 is a schematic diagram of a data acquisition system of thescanning system of FIG. 14;

FIG. 18 is a partial view of a dual energy scanner according to afurther embodiment of the invention;

FIG. 19 is a further partial view of the scanner of FIG. 18;

FIG. 20 is a schematic view of a dual energy X-ray source of a furtherembodiment of the invention;

FIG. 21 is a schematic view of a detector array of a scanner accordingto a further embodiment of the invention;

FIG. 22 is a schematic view of a detector array of a scanner accordingto a further embodiment of the invention;

FIG. 23 is a circuit diagram of a data acquisition circuit of theembodiment of FIG. 21;

FIG. 24 is a circuit diagram of a data acquisition circuit of a furtherembodiment of the invention; and

FIG. 25 is one embodiment of a baggage handling system with a diversionloop;

FIG. 26 is another embodiment of a baggage handling system with adiversion loop;

FIG. 27 is one embodiment of a confirmatory sensor comprising a NQRdetection system;

FIG. 28a is a first exemplary graph of a pulse sequence for an NQRdetection system;

FIG. 28b is a second exemplary graph of a pulse sequence for an NQRdetection system;

FIG. 29 is an example X-ray spectrum from a standard X-ray tube;

FIG. 30a is a first exemplary indicative X-ray diffraction spectra foran amorphous material like water and a polycrystalline material such asan explosive;

FIG. 30b is a second exemplary indicative X-ray diffraction spectra foran amorphous material like water and a polycrystalline material such asan explosive;

FIG. 31 is an exemplary embodiment of an X-ray diffraction system foruse as a confirmation sensor;

FIG. 32 is an inspection region defined by the intersection of a primarybeam and a secondary collimator beam;

FIG. 33 depicts a detector coupled to a suitable readout circuit withpulse shaping capability;

FIG. 34 depicts a diffraction detection package arranged to liesubstantially parallel to the direction of conveyor motion;

FIG. 35 depicts a boom fixed to a control system that allows thediffraction beam to be rotated from a substantially vertical orientationto a substantially horizontal orientation;

FIG. 36 depicts an exemplary backscatter system; and

FIG. 37 depicts an exemplary trace detection system.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 to 3, a concourse baggage scanning system 6comprises a scanning unit 8 comprising a multi-focus X-ray source 10 andX-ray detector array 12. The source 10 comprises a large number ofsource points 14 in respective spaced locations on the source, andarranged in a full 360.degree circular array around the axis X-X of thesystem. It will be appreciated that arrays covering less than the full360.degree. angle can also be used.

Referring to FIG. 1a , the X-ray source 10 is made up of a number ofsource units 11 which are spaced around the scanning region 16 in asubstantially circular arrangement, in a plane perpendicular to thedirection of movement of the conveyor. Each source unit 11 comprises aconductive metal suppressor 13 having two sides and an emitter element15 extending along between the suppressor sides. A number of gridelements in the form of grid wires 17 are supported above the suppressor13 perpendicular to the emitter element 15. A number of focusingelements in the form of focusing wires 19 are supported in another planeon the opposite side of the grid wires to the emitter element. Thefocusing wires 19 are parallel to the grid wires 17 and spaced apartfrom each other with the same spacing as the grid wires, each focusingwire 19 being aligned with a respective one of the grid wires 17.

The focusing wires 19 are supported on two support rails 21 which extendparallel to the emitter element 15, and are spaced from the suppressor13. The support rails 21 are electrically conducting so that all of thefocusing wires 19 are electrically connected together. One of thesupport rails 21 is connected to a connector 23 to provide an electricalconnection for the focusing wires 19. Each of the grid wires 17 extendsdown one side of the suppressor 12 and is connected to a respectiveelectrical connector 25 which provide separate electrical connectionsfor each of the grid wires 17.

An anode 27 is supported above the grid wires 17 and focusing wires 19.The anode 27 is formed as a rod, typically of copper with tungsten orsilver plating, and extends parallel to the emitter element 15. The gridand focusing wires 17, 19 therefore extend between the emitter element15 and the anode 27. An electrical connector 29 provides an electricalconnection to the anode 27.

The grid wires 17 are all connected to a negative potential, apart fromtwo which are connected to a positive potential. These positive gridwires extract a beam of electrons from an area of the emitter element 15and, with focusing by the focusing wires 19, direct the electron beam ata point on the anode 27, which forms the X-ray source point for thatpair of grid wires. The potential of the grid wires can therefore beswitched to select which pair of grid wires is active at any one time,and therefore to select which point on the anode 27 is the active X-raysource point at any time.

The source 10 can therefore be controlled to produce X-rays from each ofthe source points 14 in each of the source units 11 individually and,referring back to FIG. 1, X-rays from each source point 14 are directedinwards through the scanning region 16 within the circular source 10.The source 10 is controlled by a control unit 18 which controls theelectrical potentials applied to the grid wires 17 and hence controlsthe emission of X-rays from each of the source points 14. Other suitableX-ray source designs are described in WO 2004/097889.

The multi-focus X-ray source 10 allows the electronic control circuit 18to be used to select which of the many individual X-ray source points 14within the multi-focus X-ray source is active at any moment in time.Hence, by electronically scanning the multi-focus X-ray tube, theillusion of X-ray source motion is created with no mechanical partsphysically moving. In this case, the angular velocity of source rotationcan be increased to levels that simply cannot be achieved when usingconventional rotating X-ray tube assemblies. This rapid rotationalscanning translates into an equivalently speeded up data acquisitionprocess and subsequently fast image reconstruction.

The detector array 12 is also circular and arranged around the axis X-Xin a position that is slightly offset in the axial direction from thesource 10. The source 10 is arranged to direct the X-rays it producesthrough the scanning region 16 towards the detector array 12 on theopposite side of the scanning region. The paths 18 of the X-ray beamstherefore pass through the scanning region 16 in a direction that issubstantially, or almost, perpendicular to the scanner axis X-X,crossing each other near to the axis. The volume of the scanning regionthat is scanned and imaged is therefore in the form of a thin sliceperpendicular to the scanner axis. The source is scanned so that eachsource point emits X-rays for a respective period, the emitting periodsbeing arranged in a predetermined order. As each source point 14 emitsX-rays, the signals from the detectors 12, which are dependent on theintensity of the X-rays incident on the detector, are produced, and theintensity data that the signals provide are recorded in memory. When thesource has completed its scan the detector signals can be processed toform an image of the scanned volume.

A conveyor belt 20 moves through the imaging volume, from left to right,as seen in FIG. 1, parallel to the axis X-X of the scanner. X-rayscatter shields 22 are located around the conveyor belt 20 upstream anddownstream of the main X-ray system to prevent operator dose due toscattered X-rays. The X-ray scatter shields 22 include lead rubber stripcurtains 24 at their open ends such that the item 26 under inspection isdragged through one curtain on entering, and one on leaving, theinspection region. In the integrated system shown, the main electroniccontrol system 18, a processing system 30, a power supply 32 and coolingracks 34 are shown mounted underneath the conveyor 20. The conveyor 20is arranged to be operated normally with a continuous scanning movementat constant conveyor speed, and typically has a carbon-fiber frameassembly within the imaging volume.

Referring to FIG. 4 the processing system 30 includes an electronic dataacquisition system and real-time image reconstruction system. The X-raydetector array 12 comprises banks of individual X-ray detectors 50configured in a simple linear pattern (e.g. 1.times.16). Multiple ringpatterns (e.g. 8.times.16) are also possible. Each detector 50 outputs asignal dependent on the intensity of the X-rays it detects. Amultiplexing block 52 multiplexes the output data signals from each ofthe input X-ray detectors 50, performs data filtering, gain and offsetcorrections and formats the data into a high-speed serial stream. Aselection block 53 takes input from all of the multiplexing blocks 52and selects just the part of the entire X-ray data that is required forthe image reconstruction. The selection block 53 also determines theun-attenuated X-ray beam intensity, Io, for the appropriate X-ray sourcepoint (which will vary for every X-ray source point within themulti-focus X-ray tube), processes the X-ray intensity data, Ix, fromthe multiplexing block 52 by forming the result log_(o)(Ix/Io) and thenconvolves this with a suitable 1-D filter. The resulting projection datais recorded as a sinogram, in which the data is arranged in an arraywith pixel number along one axis, in this case horizontally, and sourceangle along another axis, in this case vertically.

Data is then passed from the selection block 53 in parallel to a set ofback projection-summation processor elements 54. The processor elements54 are mapped into hardware, using look-up tables with pre-calculatedcoefficients to select the necessary convolved X-ray data and weightingfactors for fast back projection and summation. A formatting block 55takes the data representing individual reconstructed image tiles fromthe multiple processor elements 54 and formats the final output imagedata to a form suitable for generating a suitably formatted threedimensional image on a display screen. This output can be generated fastenough for the images to be generated in real time, for viewing in realtime or off-line, hence the system is termed a real time tomography(RTT) system.

In this embodiment the multiplexing block 52 is coded in software, theselection block 53 and formatting block 55 are both coded in firmware,and the processor elements mapped in hardware. However, each of thesecomponents could be either hardware or software depending on therequirements of the particular system.

Referring to FIG. 5 each of the final output images for each baggageitem is then processed by a threat detection processor 60 within theprocessing system 30 which is arranged to determine whether the imagedbaggage item represents a threat. In the threat detection processor 60,input X-ray tomographic image data 62 is passed in to a set of low-levelparameter extractors 63 (level 1). The parameter extractors 63 identifyfeatures in the image such as areas of constant grey level, texture andstatistics. Some of the extractors work on the data for individual 2dimensional images or slices, some work on the 3 dimensional images, andsome work on the sinogram data. Where possible, each extractor works inparallel on the same set of input data, and each extractor is arrangedto perform a different processing operation and to determine a differentparameter. At the end of the processing, the parameters determined bythe parameter extractors 63 are passed up to a set of decision trees 64(level 2). Details of the parameters extracted are given below. Thedecision trees 64 each take a number (typically all) of the low levelparameters and construct respective higher level information, such asinformation regarding contiguous volumes, with associated statistics. Atthe top level (level 3), a database searcher 65 maps the higher levelparameters produced at level 2 into a ‘red’ probability Pr(threat) ofthere being a threat present and a ‘green’ probability Pr(safe) of theitem under inspection being safe. These probabilities are used by theprocessing system 30 to allocate the scanned item to an appropriatesafety category, and to produce an automatic sorting control output.This automatic sorting control output can be either a first ‘green’output indicating that the item is allocated to a clear category, asecond ‘red’ output indicating that the item is allocated to a ‘notclear’ category, or a third ‘amber’ output indicating that the automaticsorting cannot be carried out with sufficient reliability to allocatedthe item to the ‘clear’ or the ‘not clear’ category. Specifically ifPr(safe) is above a predetermined value, (or Pr(threat) is below apredetermined value) then the automatic sorting output will be producedhaving a first signal form, indicating that the item should be allocatedto the green channel. If Pr(threat) is above a predetermined value, (orPr(safe) is below a predetermined value) then the automatic sortingoutput will be produced having a second signal form, indicating that theitem should be allocated to the red channel. If Pr(threat) (or Pr(safe))is between the two predetermined values, then the automatic sortingoutput is produced having a third signal form, indicating that the itemcannot be allocated to either the red or green channel. Theprobabilities can also be output as further output signals.

The parameters that will be determined by the parameter extractors 63generally relate to statistical analysis of pixels within separateregions of the 2-dimensional or 3-dimensional image. In order toidentify separate regions in the image a statistical edge detectionmethod is used. This starts at a pixel and then checks whether adjacentpixels are part of the same region, moving outwards as the region grows.At each step an average intensity of the region is determined, bycalculating the mean intensity of the pixels within the region, and theintensity of the next pixel adjacent to the region is compared to thatmean value, to determine whether it is close enough to it for the pixelto be added to the region. In this case the standard deviation of thepixel intensity within the region is determined, and if the intensity ofthe new pixel is within the standard deviation, then it is added to theregion. If it is not, then it is not added to the region, and thisdefines the edge of the region as being the boundary between pixels inthe region and pixels that have been checked and not added to theregion.

Once the image has been divided into regions, then parameters of theregion can be measured. One such parameter is a measure of the varianceof the pixel intensity within the region. If this is high this might beindicative of a lumpy material, which might for example be found in ahome-made bomb, while if the variance is low this would be indicative ofa uniform material such as a liquid.

Another parameter that is measured is the skewedness of the distributionof pixel value within the region, which is determined by measuring theskewedness of a histogram of pixel values. A Gaussian, i.e. non-skewed,distribution indicates that the material within the region is uniform,whereas a more highly skewed distribution indicates non-uniformities inthe region.

As described above, these low-level parameters are passed up to thedecision trees 64, where higher level information is constructed andhigher level parameters are determined. One such higher level parameteris the ratio of the surface area to the volume of the identified region.Another is a measure of similarity, in this case cross-correlation,between the shape of the region and template shapes stored in thesystem. The template shapes are arranged to correspond to the shape ofitems that pose a security threat, such as guns or detonators. Thesehigh level parameters are used as described above to determine a level1f threat posed by the imaged object.

Referring to FIG. 6 an in-line real time tomography baggage sortingsystem comprises the scanning system 6 of FIG. 1 with the conveyor 20passing through it. Downstream of the scanning system 6 a sorting device40 is arranged to receive articles of baggage from the conveyor 20 andmove them onto either a clear or ‘green’ channel conveyor 42 or a notclear or ‘red’ channel conveyor 44. The sorting device 40 is controlledby the automatic sorting output signals via a control line 46 from theprocessing system 30, which are indicative of the decision of theprocessing system 30 as to whether the item is clear or not, and also bysignals from a workstation 48 to which it is connected via line 45. Theimages from the scanning system 6 and signals from the processing system30, indicative of the red and green probabilities and the nominaldecision of the processing system 30, are also fed to the workstation48. The workstation is arranged to display the images on a screen 47 sothat they can be viewed by a human operator, and also to provide adisplay indicative of the green and red probabilities and the nominalautomatic sorting decision. The user at the workstation can review theimages and the probabilities, and the automatic sorting output, anddecide whether to accept or override the decision of the scanningsystem, if that was to allocate the item to the red or green category,or to input a decision if the scanning system decision was to allocatethe item to the ‘amber’ category. The workstation 48 has a user input 49that enables the user to send a signal to the sorting device 40 whichcan be identified by the sorting device as over-riding the decision ofthe scanning system. If the over-riding signal is received by thesorting device, then the sorting device does over-ride the decision ofthe scanning system. If no over-ride signal is received, or indeed if aconfirming signal is received from the workstation confirming thedecision of the scanning system, then the sorting device sorts the itemon the basis of the decision of the scanning system. If the sortingsystem receives an amber′ signal from the scanning system relating to anitem, then it initially allocates that item to the ‘red’ category to beput into the red channel. However, if it receives an input signal fromthe workstation before it sorts the item indicating that it should be inthe ‘green’ category, then it sorts the item to the green channel.

In a modification to the system of FIG. 6, the sorting can be fullyautomatic, with the processing system giving one of just two sortingoutputs, ‘clear’ and ‘not clear’, allocating the item to either thegreen or the red channel. It would also be possible for the processingsystem to determine just one probability Pr(threat) with one thresholdvalue and allocate the item to one of the two categories depending onwhether the probability is above or below the threshold. In this casethe allocation would still be provisional and the operator would stillhave the option of overriding the automatic sorting. In a furthermodification the automatic category allocation of the scanning system isused as the final allocation, with no user input at all. This provides afully automated sorting system.

In the system of FIG. 6, the scan speed is matched to the conveyorvelocity, so that the baggage can be moved at a constant velocity from aloading area where it is loaded onto the conveyor 20, through thescanning system 6, and on to the sorting device 40. The conveyor 20extends for a distance L, between the exit of the scanning system 6 andthe sorting device 40. During the time that a baggage item takes totravel the distance L on the conveyor 20, an operator can view the imagedata of the item under inspection, and the initial category allocationdetermined by the scanning system, and confirm or reject the automateddecision of the RTT system. Typically the baggage would then be eitheraccepted into the Clear channel and passed forward ready fortransportation or rejected into the Not Cleared channel for furtherinvestigation.

In this RTT multi-focus system, the RTT scanning unit 8 is able tooperate at full baggage belt speed, and hence no baggage queuing orother divert mechanism is required for optimal system operation. Inintegrated systems such as this, the limited throughput capability ofconventional rotating source systems is a significant constraint. Oftenthis means placing multiple conventional CT machines in parallel, andusing sophisticated baggage handling systems to switch the item forinspection to the next available machine. This complexity can be avoidedwith the arrangement of FIG. 6.

Referring to FIG. 7 a second embodiment of the invention comprises aredundant system in which two RTT scanning systems 70, 72 are located inseries on the same conveyor 74 such that if one system were to be takenout of service, then the other could continue to scan baggage. In eithercase, the conveyor belt 74 would continue to run through both scanningsystems 70, 72 at standard operating belt speed.

Referring to FIG. 8a in a third embodiment there is provided a morecomplex redundant system in which two RTT systems 82, 84 are operated inparallel. A first main incoming conveyor 86 brings all items to besorted to a first sorting device 88 which can transfer items onto eitherone of two further conveyors 90, 92. Each of these two conveyors 90, 92passes through a respective one of the scanning systems 82, 84, whichwill scan the items and enable a decision to be made as to whether toclear the item or not. A further sorting device 94, 96 is provided oneach of the two conveyors 90, 92 which is arranged to sort the baggageonto a common ‘green channel’ conveyor 98 for onward transportation, ora ‘red channel’ conveyor 100 if it is not cleared, where it can undergofurther investigation. In this configuration, it is possible to run theinput conveyor 86, and the ‘green channel’ conveyor at a higher speedthan the RTT conveyor speed, typically up to twice the speed. Forexample in this case the main incoming conveyor 86 and the common ‘greenchannel’ conveyor move at a speed of 1 m/s whereas the scanningconveyors 82, 84 travel at half that speed, i.e. 0.5 m/s. Of course thesystem can be expanded with more parallel RTT systems, with the ratio ofthe speed of the main incoming conveyor to that of the scanner conveyorsbeing equal to, or substantially equal to, the number of parallelscanners, although the sorting devices may become unreliable at morethan about 1 m/s main conveyor speed.

Referring to FIG. 8b , in a further embodiment a baggage sorting systemcomprises a number of RTT scanners 81 b, 82 b, 83 b, typically up toabout 60 in one system, each associated with a respective check-in desk.A sorting device 84 b, 85 b, 86 b is associated with each RTT scanner,and baggage is conveyed on a conveyor from each RTT scanner to itsassociated sorting device. Each sorting device 84 b, 85 b, 86 b sortsthe baggage, in response to signals from its scanner, onto either acommon clear channel conveyor 88 b, or a common reject channel conveyor87 b. A further backup RTT scanner 89 b is provided on the rejectchannel conveyor 87 b, with an associated sorting device 90 b, that canleave baggage on the reject channel conveyor 87 b, or transfer it to theclear channel conveyor 88 b.

Under normal operation, each of the primary scanners 81 b, 82 b, 83 bsorts the baggage, and the backup or redundant scanner 89 b simplyprovides a further check on items sorted into the reject channel. Ifthat scanner determines that an item of baggage represents no, or asufficiently low threat, then it transfers it to the clear channel. Ifone of the primary scanners is not functioning or has a fault, then itsassociated sorting device is arranged to sort all baggage from thatscanner to the reject channel. Then, the back-up scanner 89 b scans allof that baggage and controls sorting of it between the clear and rejectchannels. This enables all the check-in desks to continue to functionwhile the faulty scanner is repaired or replaced.

Referring to FIG. 8c , in a further embodiment, baggage from each of thecheck-in desks is transferred via a plurality of separate conveyors ontoa central loop or carousel 81 c, on which it circulates continuously. Anumber of sorting devices 82 c, 83 c, 84 c are each arranged to transferitems of baggage from the loop 81 c to a respective conveyor leading toa respective RTT scanner 85 c, 86 c, 87 c. The sorting devices 82 c, 83c, 84 c are controlled by the scanners to control the rate at whichbaggage items are fed to each of the scanners. From the scanners,conveyors transfer all of the baggage items to a common exit conveyor 88c leading to a further sorting device 89 c. This is controlled by all ofthe scanners to sort each of the baggage items between a clear channel90 c and a reject channel 91 c.

In order to track the movement of each item of baggage, each item isgiven a 6-digit ID, and its position on the conveyor recorded when itfirst enters the system. The scanners can therefore identify which itemof baggage is being scanned at any one time, and associate the scanningresults with the appropriate item. The sorting devices can thereforealso identify the individual baggage items and sort them based on theirscanning results.

The number of scanners and the speeds of the conveyors in this systemare arranged such that, if one of the scanners is not functioning, theremaining scanners can process all of the baggage that is being fed ontothe loop 81 c from the check-in desks.

In a modification to this embodiment, the sorting devices 82 c, 83 c, 84c that select which items are transferred to each scanner are notcontrolled by the scanners, but are each arranged to select items fromthe loop 81 c so as to feed them to the respective scanner at apredetermined rate.

Referring to FIG. 9 a networked system according to a further embodimentcomprises three scanning systems 108 similar to that of FIG. 6, and fouroperator workstations 148. The video image outputs from the three RTTscanning systems 108 are connected via respective high bandwidthpoint-to-point video links to real time disk arrays 109 which providingtransient storage for the raw image data, to a redundant video switch110. The disk arrays 109 are in turn connected to each of theworkstations 148. The video switch 110 is therefore able to transmit theraw video image output from each of the scanning systems 108 from itstemporary storage, to any one of the workstations 148, where it can beused to create 3-dimensional video images which can be viewed off-line.The outputs from the scanning system for the red/green probabilitysignals and the automatic sorting allocation signals are connected to aredundant conventional Ethernet switch 112, which is also connected toeach of the workstations. The Ethernet switch is arranged to switch eachof the probability signals and the sorting allocation signals to thesame workstation 148 as the associated video signal. This allows imagedata from the multiple machines together with the automatic allocationand probabilities assigned to the allocation, to be switched through tothe bank of operator workstations 148 where an operator can both monitorthe performance of the baggage inspection system and determine thedestination of baggage assigned an amber threat level.

Alternatively, a networked system comprises a single scanning system 108connected to a server and a workstation 148. The video image output fromthe scanning system 108 is connected to a real time disk array 109,which provides transient storage for the raw image data. The disk array109 is in turn connected to the workstation 148. The probability signaland allocation signal outputs are sent to the workstation 148 togetherwith the associated video image output to be monitored by an operator.The networked single scanning system may be part of a networked systemwith multiple scanning systems.

Referring to FIGS. 10 and 11, in a further embodiment an in-line scannerhas a conveyor belt 160 just as long as the main scatter shields 162. Insuch standalone system configurations, the item for inspection is placedonto the conveyor belt 160 and the item loaded into the system. The itemis then scanned through the scanner machine 164 and images aregenerated. Often, in conventional systems, the item is pre-screened witha simple transmission X-ray system to identify likely threat areas priorto computed tomography screening of selected planes in the object. Suchapplications are simple for a real-time multi-focus system to cope with.Here, no pre-screening would be used and a true three-dimensional imageof the complete item would be obtained.

In some embodiments the locus of source points in the multi-focus X-raysource will extend in an arc over an angular range of only 180 degreesplus the fan beam angle (typically in the range 40 to 90 degrees). Thenumber of discrete source points is advantageously selected to satisfythe Nyquist sampling theorem. In some embodiments, as in that of FIG. 1,a complete 360 degree ring of source points is used. In this case, thedwell-time per source point is increased over a 180+ fan beamconfiguration for a given scan rate and this is advantageous inimproving reconstructed image signal-to-noise ratio.

The scanner system of FIG. 1 is an integrated scanner system, in thatthe control, processing, power supply, and cooling units 18, 30, 32, 34are housed in a unit with the scanning system 8 and the screening 22.Referring to FIG. 12, in a further embodiment there is provided amodular system in which some, or all, of the control, processing, powersupply, and cooling racks 218, 230, 232, 234 are located remotely fromthe scanning unit 208 comprising multi-focus X-ray source and sensorarray. It is advantageous to use a modular design to facilitate easyinstallation, particularly in baggage handling hall environments, wheresystems may be suspended from the ceiling or in regions with restrictedaccess. Alternatively, a complete system can be configured as anintegrated unit with the sub-assembly units co-located within a singlehousing.

In some embodiments, including that of FIG. 1, a single X-ray detectorring is used. This is inexpensive to construct and provides adequatesignal-to-noise performance even at high image scanning rates with asimple fan-beam image reconstruction algorithm. In other embodiments(particularly for large image reconstruction circle diameter) it ispreferable to use a multi-ring sensor array with a plurality of circularor part-circular groups of sensors arranged adjacent to each other,spaced along the axis of the system offset from the source. This enablesa more complex cone-beam image reconstruction algorithm to be used inthe processing system. The use of a multi-ring sensor increasesdwell-time per source point resulting in larger integrated signal sizeand consequent improvement in signal-to-noise ratio in the reconstructedimage.

Central to the design of the embodiments described above, which use amulti-focus X-ray source based computed tomography system, is therelationship between the angular rotational speed of the source and thevelocity of the conveyor system passing through the scanner. In thelimit that the conveyor is stationary, the thickness of thereconstructed image slice is determined entirely by the size of theX-ray focus and the area of the individual elements of the X-raydetector array. As conveyor speed increases from zero, the object underinspection will pass through the imaging slice during rotation of theX-ray beam and an additional blurring will be introduced into thereconstructed image in the direction of the slice thickness. Ideally,the X-ray source rotation will be fast compared to the conveyor velocitysuch that blurring in the slice thickness direction will be minimized.

A multi-focus X-ray source based computed tomography system for baggageinspection provides a good ratio of angular source rotational speed tolinear conveyor speed for the purposes of high probability detection ofthreat materials and objects in the item under inspection. As anexample, in the embodiment of FIG. 1, the conveyor speed is 0.5 m/s asis common in airport systems. The source can achieve 240 sourcerotations about the conveyor per second, so the object under inspectionwill move a distance of 2.08 mm through the imaging slice during thescan. In a conventional system with source rotation of 4 revolutions persecond, the object under inspection will move a distance of 62.5 mmthrough the imaging slice during the scan at the same belt speed.

The primary goal of an inspection system for detection of threatmaterials is to detect accurately the presence of threat materials andto pass as not suspect all other materials. The larger the blurring inthe slice direction that is caused by conveyor motion during a scan, thegreater the partial volume artifact in the reconstructed image pixel andthe less accurate the reconstructed image density. The poorer theaccuracy in the reconstructed image density, the more susceptible thesystem is to provide an alarm on non-threat materials and to not raisean alarm on true threat materials. Therefore, a real-time tomography(RTT) system based on multi-focus X-ray source technology can provideconsiderably enhanced threat detection capability at fast conveyorspeeds than conventional mechanically rotated X-ray systems.

Due to the use of an extended arcuate anode in a multi-focus X-raysource, it is possible to switch the electron source such that it jumpsabout the full length of the anode rather than scanning sequentially toemulate the mechanical rotation observed in conventional computedtomography systems. Advantageously, the X-ray focus will be switched tomaximize the distance of the current anode irradiation position from allprevious irradiation positions in order to minimize the instantaneousthermal load on the anode. Such instantaneous spreading of the X-rayemission point is advantageous in minimizing partial volume effect dueto conveyor movement so further improving reconstructed pixel accuracy.

The high temporal resolution of RTT systems allows a high level ofaccuracy to be achieved in automated threat detection. With this highlevel of accuracy, RTT systems can be operated in unattended mode,producing a simple two-state output indication, with one statecorresponding to a green or clear allocation and the other to a red ornot clear allocation. Green bags are cleared for onward transport. Redbags represent a high level of threat and should be reconciled with thepassenger and the passenger barred from traveling.

Further embodiments of the invention will now be described in which datarelating to the scattering of X-rays as well as that relating totransmitted X-rays is recorded and used to analyze the scanned baggageitems.

Referring to FIG. 13 when a beam 300 of X-rays passes through an object302, some of the X-rays are transmitted straight through it, and exitthe object traveling in the same direction as they entered it. Some ofthe X-rays are scattered through a scattering angle θ, which is thedifference between the direction in which they enter the object and thedirection in which they leave it. As is well known there are two typesof scattering that occur: coherent or Bragg scattering, which isconcentrated around scattering angles of 5 degrees, typically in therange 4 degrees to 6 degrees, and incoherent or Compton scattering inwhich the X-ray is scattered through larger angles. Bragg scatteringincreases linearly with the atomic number of the object and obeys theformula:

nλ=2d sin θ

where n is an integer, λ is the wavelength of the X-ray, and d is theinter-atomic distance in the object.

Therefore the amount of Bragg scattering gives information about theatomic structure of the object. However, it does not vary smoothly withatomic number.

The amount of Compton scattering is dependent on, and varies smoothlywith, the electron density of the object, and therefore the amount ofscattering at higher scatter angles gives information about the electrondensity of the object, and hence about its atomic number.

Referring to FIG. 14 a security scanning system according to a furtherembodiment of the invention comprises a multi-focus X-ray source 410which is the same as that of FIG. 1, and a circular detector array 412and conveyor 420 that are also the same as those of FIG. 1. However, inthis embodiment, the system comprises a further cylindrical array ofdetectors 422 which also extends around the conveyor at the same radiusas the circular detector array 412 but on the other side axially of thesource 410. Whereas the circular detector array is arranged to detectX-rays transmitted through the object 426, the cylindrical detectorarray 422 is arranged to detect X-rays scattered in the object. Thescatter detector array 422 is made up of a number of circular arrays orrings 422 a, 422 b of detectors, and the detectors in each ring areequally spaced around the conveyor so that they are arranged in a numberof straight rows extending in the axial direction of the scanner.

The detectors in the scatter detector array 422 are energy resolvingdetectors such that individual X-ray interactions with each detectorproduce a detector output that is indicative of the energy of the X-ray.Such detectors can be fabricated from wide bandgap III-V or II-IVsemiconductor materials such as GaAs, HgI, CdZnTc or CdTc, a narrow gapsemiconductor such as Ge, or a composite scintillation detector such asNaI(Ti) with photomultiplier tube readout.

Referring to FIG. 15, a collimator 428 is provided in front of thescattering detectors 422. The collimator 428 provides a barrier thatprevents X-rays from reaching each detector unless it comes from aparticular receiving direction. For each detector in the array 422, thereceiving direction passes through the central longitudinal axis X-X ofthe scanner, as can be seen in FIG. 16. However, the receiving directionis not perpendicular to the axis X-X, but is inclined at about 5.degree.to the plane of the detector rings 422 a, 422 b in the direction towardsthe source 410, as can be seen in FIG. 15.

Referring to FIG. 15 it will be appreciated that X-rays incident on anyone of the detectors of the array 422 must have been scattered from arespective small sub-volume within the thin imaged volume that lies bothin the path of the X-ray beam and in the line of the receiving directionfrom the detector 422. For any coherently scattered X-rays, the axialposition of the detector that detects it will be determined by thedistance from the active X-ray source point at which the scatteringoccurred. Detectors nearest the source 410 in the axial direction willdetect X-rays scattered furthest from the active X-ray source point. Forexample X-rays scattered from the point x, which is nearest the activeX-ray source point 410 a, will be detected by a detector further fromthe source 410 than X-rays scattered from the point z which is furtherfrom the active X-ray source point. Therefore, at any one time, when theactive X-ray source point can be identified, the axial position of thedetector which detects the scattered X-ray can be used to determine theposition of the scattering along the X-ray beam direction.

It will also be appreciated from FIG. 15 that, for this system to work,it is important that the X-ray beam should be narrowly focused in theaxial direction of the scanner. Spreading of the beam in the transversedirection, e.g. use of a fan beam spread in the transverse directionwill still allow this positioning of coherent scattering events.

Referring to FIG. 16, because the collimator 428 is directed towards theaxis of the scanner, X-rays from an active source point 410 a thatundergo coherent scattering will only be detected by the row ofdetectors 422 a that is on the opposite side of the scanner axis to theactive source point, and possibly one or more of the rows close to it oneither side depending on how narrowly focused the collimator is. IfX-rays are confined to a straight narrow ‘pencil’ beam, then any X-raysthat are scattered incoherently through larger angles will not bedetected at all as they will be cut off by the collimator 428. Anexample of such an X-ray is shown by arrow ‘a’ in FIG. 16. However, if afan beam of X-rays is produced from the active source point 410 a, thatis spread out through the imaging volume slice in the directionperpendicular to the scanner axis, then X-rays directed further awayfrom the scanner axis can undergo incoherent scattering and reachdetectors to either side of the row 422 a opposite the active sourcepoint. Examples of such X-rays are shown by the arrows b and c. It willbe noted that, to reach any detector 422 b, the scattering event musttake place in the plane passing through the scanner axis and thatdetector 422 b. This means that, for a given active source point and aparticular detector, the position of the scattering event of a detectedX-ray can be identified as being in the plane passing through thescanner axis and that detector. If the exact position of the scatteringevent is to be determined then other information is needed. For exampleif information regarding the position of objects within the imagingvolume is available, for example from tomographic imaging data, then thescattering can be associated with the most likely object as will bedescribed in more detail below.

From the Bragg scattering data, for each detected scattering event, thecombination of the X-ray energy and the scatter angle can be used todetermine the inter-atomic distance d of the material in which thescattering event took place. In practice, the scatter angle can beassumed to be constant, and the energy used to distinguish betweendifferent materials. For the Compton scattering, the level of scatteringfrom each volume of the scanning volume gives an indication of thedensity of the material in that volume. The ratio of Compton to coherentscatter can also be determined and used as a further parameter tocharacterize the material of the imaged object.

Due to the short dwell time for each X-ray source point, the number ofdetected scattered X-rays for each source point will always be very low,typically less than five. In order to form a reasonable coherent scattersignal it is necessary to collect scatter data for all source pointswithin a tomographic scan and then accumulate the results for eachsub-volume of the imaging volume. For a scanner with 500 source points,and an average of one coherent diffraction scatter result per sub-volumeper scan, then following accumulation of the set of data, eachsub-volume will have 500 results associated with it, corresponding to500 scattering events within that sub-volume. A typical sub-volumeoccupies an area within the imaging plane of a few square centimeters,with a volume thickness of a few millimeters.

Referring to FIG. 17, the data acquisition system arranged to accumulatedata from the scatter detector array 422 of the scanner of FIGS. 14 to16 comprises a multi-channel analyzer 500 associated with each of thedetectors 422. Each MCA 500 is arranged to receive the output signalsfrom the detector, and allocate each X-ray detected to one of a numberof X-ray energy ranges or channels, and output a signal indicative ofthe energy range in which the detected X-ray falls. A multiplexer 502 isarranged to receive the outputs from each of the MCAs 500. A look-uptable 504 is also provided which has entries in it that, for a givensource point and detector, identify the sub-volume within the imagingvolume in which the X-ray was scattered. The system further comprises animage memory 506 which includes a number of memory areas 508, each ofwhich is associated with a respective sub-volume within the scannerimaging plane.

Data is loaded into each memory area 508 automatically by themultiplexer 502 under the direction of the look up table 504. The lookup table is loaded with coefficients prior to scanning that map eachcombination of detector 422 and MCA 500 to a respective image location508, one look up table entry per X-ray source position. Those pixels,i.e. detectors 422, that are in the forward direction, i.e.substantially in the direction that the photon is traveling from thesource prior to any interaction, are assumed to record coherent scatterphotons at small beam angles of about 4-6 degrees. Those pixels 422 thatare not in the forward direction are assumed to record incoherentscattered photons due to the Compton scattering effect. Hence, the imagememory 506 is actually “three dimensional”—two dimensions representlocation in the image while the third dimension holds scattered energyspectra for both coherent (lo 8-bits) and incoherent scattering (hi 8bits). The look up table 504 will also instruct the multiplexer 502 asto the type of data that is being collected for each MCA 500 at eachprojection so that the appropriate memory space is filled.

Once the scatter data has been collected for a given scan, the data istransferred to and synchronized, by a projection sequencer 510, with themain RTT data acquisition system 512, which is described above withreference to FIG. 4. Hence the reconstructed image data and scatter dataare passed through simultaneously to the threat detection system, whichcan use it to determine suitable parameters for analysis.

For each scan, the tomographic image data from the transmissiondetectors 412 produces data relating to the X-ray attenuation for eachpixel of the image, which in turn corresponds to a respective sub-volumeof the tomographic imaging volume. This is obtained as described abovewith reference to FIG. 4. The data from the scatter detectors 422provides, as described above, data relating to the amount of coherentscattering within each sub-volume, and data relating to the amount ofincoherent scattering within each sub-volume. This data can therefore beanalyzed in a threat detection processor similar to that of FIG. 5. Inthis case the parameters of the data which are extracted can relate tothe image data or the scatter data or combinations of two or more typesof data. Examples of parameters that are extracted from the data are theratio of coherent to incoherent scatter, material types as determinedfrom coherent scatter data, material density as determined fromincoherent scatter data, correlation of CT image pixel values withscatter data. Also parameters for the scatter data corresponding tothose described above for the transmission data can also be determined.

Referring to FIG. 18, in a further embodiment of the invention thetransmission detectors 512 that are used to generate the tomographicimage data are arranged to measure the X-ray transmission over differentenergy ranges. This is achieved by having two sets of detectors 512 a,512 b, each forming a ring around the conveyor. The two sets are atdifferent axial locations along the direction of travel of the conveyor,in this case being adjacent to each other in the axial direction. Thefirst set 512 a has no filter in front of it, but the second set 512 bhas a metal filter 513 placed between it and the X-ray source 510. Thefirst set of detectors 512 a therefore detects transmitted X-rays over abroad energy range, and the second set 512 b detects X-rays only in anarrower part of that range at the high energy end.

As the item to be scanned moves along the conveyor, each thin volume orslice of it can be scanned once using the first set of detectors 512 aand then scanned again using the second set 512 b. In the embodimentshown, the same source 510 is used to scan two adjacent volumessimultaneously, with data for each of them being collected by arespective one of the detector sets 512 a, 512 b. After a volume of theitem has moved past both sets of detectors and scanned twice, two setsof image data can be formed using the two different X-ray energy ranges,each image including transmission (and hence attenuation) data for eachpixel of the image. The two sets of image data can be combined bysubtracting that for the second detector set 512 a from that of thefirst 512 b, resulting in corresponding image data for the low energyX-ray component.

The X-ray transmission data for each individual energy range, and thedifference between the data for two different ranges, such as the highenergy and low energy, can be recorded for each pixel of the image. Thedata can then be used to improve the accuracy of the CT images. It canalso be used as a further parameter in the threat detection algorithm.

It will be appreciated that other methods can be used to obtaintransmission data for different ranges of X-ray energies. In amodification to the system of FIGS. 18 and 19, balanced filters can beused on the two detector sets. The filters are selected such that thereis a narrow window of energies that is passed by both of them. The imagedata for the two sets of detectors can then be combined to obtaintransmission data for the narrow energy window. This enables chemicalspecific imaging to be obtained. For example it is possible to createbone specific images by using filters balanced around the calcium K-edgeenergy. Clearly this chemical specific data can be used effectively in athreat detection algorithm.

In a further embodiment, rather than using separate filters, two sets ofdetectors are used that are sensitive to different energy X-rays. Inthis case stacked detectors are used, comprising a thin front detectorthat is sensitive to low energy X-rays but allows higher energy X-raysto pass through it, and a thick back detector sensitive to the highenergy X-rays that pass through the front detector. Again theattenuation data for the different energy ranges can be used to provideenergy specific image data.

In a further embodiment two scans are taken of each slice of the objectwith two different X-ray beam energies, achieved by using different tubevoltages in the X-ray source, for example 160 kV and 100 kV. Thedifferent energies result in X-ray energy spectra that are shiftedrelative to each other. As the spectra are relatively flat over part ofthe energy range, the spectra will be similar over much of the range.However, part of the spectrum will change significantly. Thereforecomparing images for the two tube voltages can be used to identify partsof the object where the attenuation changes significantly between thetwo images. This therefore identifies areas of the image that have highattenuation in the narrow part of the spectrum that changes between theimages. This is therefore an alternative way of obtaining energyspecific attenuation data for each of the sub-volumes within the scannedvolume.

Referring to FIG. 20 in a further embodiment of the invention, twodifferent X-ray energy spectra are produced by providing an anode 600 inthe X-ray tube that has target areas 602, 604 of two differentmaterials. In this case, for example, the anode comprises a copper base606 with one target area 602 of tungsten and one 604 of uranium. Theelectron source 610 has a number of source points 612 that can beactivated individually. A pair of electrodes 612, 614 is provided onopposite sides of the path of the electron beam 616 which can becontrolled to switch an electric field on and off to control the path ofthe electron beam so that it strikes either one or the other of thetarget areas 602, 604. The energy spectrum of the X-rays produced at theanode will vary depending on which of the target areas is struck by theelectron beam 616.

This embodiment uses an X-ray source similar to that of FIG. 1a , withthe different target areas formed as parallel strips extending along theanode 27. For each active electron source point two different X-rayspectra can be produced depending on which target material is used. Thesource can be arranged to switch between the two target areas for eachelectron source point while it is active. Alternatively the scan alongthe anode 27 can be performed twice, once for one target material andonce for the other. In either case further electron beam focusing wiresmay be needed to ensure that only one or the other of the targetmaterials is irradiated by the electron beam at one time.

Depending on the angle at which the X-ray beam is extracted from theanode, the beams from the two target areas 602, 604 can in some cases bearranged to pass though the same imaging volume and be detected by acommon detector array. Alternatively they may be arranged to passthrough adjacent slices of the imaging volume and detected by separatedetector arrays. In this case the parts of the imaged item can bescanned twice as the item passes along the conveyor in a similar mannerto the arrangement of FIG. 18.

Referring to FIG. 21, in a further embodiment, two detector arrays areprovided in a single scanner, adjacent to each other in the axialdirection, one 710 corresponding to that of FIG. 1 and being arranged toform a RTT image, and the other, 712, being of a higher resolution, andbeing arranged to produce a high resolution projection image of thescanned object. In this embodiment the high resolution detector array712 comprises two parallel linear arrays 714, 716 each arranged todetect X-rays at a different energy, so that a dual energy projectionimage can be produced. In the embodiment of FIG. 22, the high resolutionarray 812 comprises two stacked arrays, a thin array on top arranged todetect lower energy X-rays but transparent to higher energy X-rays, anda thicker array beneath arranged to detect higher energy X-rays. In bothcases, the two detector arrays are arranged close enough together in theaxial direction to be able to detect X-rays from a single linear arrayof source points.

In order to provide a projection image, data needs to be captured fromall of the detectors in the high resolution array 712, 812 when only onesource point is active. Referring to FIG. 23, in order to do this eachdetector 718, 818 in the high resolution array is connected to anintegrator 750. The integrator comprises an amplifier 752 in parallelwith a capacitor 754. An input switch 756 is provided between thedetector 718 and the amplifier 752, a reset switch 758 is providedacross the input terminals of the amplifier, and a further reset switch759 connected across the capacitor 754, and a multiplexing switch 760 isprovided between the integrator and an analogue to digital converterADC.

In operation, while the detector 718 is not required to be active, allof the switches except for the multiplexing switch 760 are closed. Thisensures that the capacitor 754 is uncharged and remains so. Then, at thestart of the period when the detector is required to gather data, thetwo reset switches 758, 759 are closed so that any X-rays detected bythe detector 718 will cause an increase in the charge on the capacitor754, which results in integration of the signal from the detector 718.When the period for data collection has ended, the input switch 756 isopened, so that the capacitor will remain charged. Then, in order forthe integrated signal to be read from the integrator, the output switch760 is closed to connect the integrator to the ADC. This provides ananalogue signal to the ADC determined by the level of charge on thecapacitor 754, and therefore indicative of the number of X-rays thathave been detected by the detector 718 during the period for which itwas connected to the integrator. The ADC then converts this analoguesignal to a digital signal for input to the data acquisition system. Toproduce a single projection image, all of the high resolution detectorsare used to collect data at the same time, when one of the X-ray sourcepoints is active.

Referring to FIG. 24, in a further embodiment, each detector 718 isconnected to two integrators 750 a, 750 b in parallel, each of which isidentical to that of FIG. 23. The outputs from the two integrators areconnected via their output switches 760 a, 760 b to an ADC. This enableseach integrator to be arranged to integrate the signal from the detector718 at a different point in the scan of the X-ray source, and thereforeto collect data for a separate image, the two images being fromdifferent angles with different X-ray source points. For example thiscan be used to produce projection images from orthogonal directionswhich can be used to build up a high resolution 3-dimensional image,from which the position of features in the imaged package can bedetermined in three dimensions.

The high resolution image can be useful when combined with the RTTimage, as it can help identify items for which higher resolution isneeded, such as fine wires.

In another embodiment of the present invention, a high speed tomographicscanner is disclosed which is capable of screening baggage and cargoitems at full conveyor speed. This is achieved by substituting themechanically scanned gantry which is used in known security screeningsystems with an electronically scanned X-ray source and associateddetection methods. The performance characteristics of such a systemprovide for automated detection of explosives and explosive devices in asingle scanning pass with follow-on human image visualisation in thosecases where the automated detection algorithm locates a suspect materialor device.

The present invention is characterised by high image quality incombination with high scanning throughput. A scanner within the scope ofthe present invention can achieve a spatial resolution of the order of 2mm or less in all three dimensions, with a reconstructed pixel size inthe three-dimensional image of 1.5 mm or less in all three dimensions.Scanners can be configured to achieve such image resolutioncharacteristics while simultaneously operating with conveyors withscanning speed of 0.25 m/s and above with a reconstructed image signalto noise ratio above 50 and generally above 100. This image qualityprovides sufficient information to unambiguously determine both thevolume and shape of potentially explosive materials with an accuracy ofmeasurement of the material linear attenuation coefficient of typically1%.

Notwithstanding the high image quality, high scanning throughput andinteractive three-dimensional image display capability of such a system,there are still occasions where an explosive material or explosivedevice is suspected and would benefit from additional screening. Inparticular, a suspect explosive material or device in a baggage or cargoitem that has been inspected using the high speed electronically scannedX-ray source scanner of the present invention can be furtherinvestigated by a secondary method to confirm the presence, or absence,of an explosive material.

In order to improve on the inspection capability of the high speed X-raysystem, a secondary sensor must probe one or more chemical properties ofthe material itself and the specific signal generated from the probemust then be correlated back to the shape, volume and expected type ofexplosive material which has been detected through the X-ray inspectionprocess.

Additionally, the detailed X-ray image must be used to target thesecondary confirmatory sensor to the specific region of the baggage orcargo item under inspection in order to maximise the performance of thesecondary sensor.

As shown in FIG. 25, a baggage handling system 2500 is generallyrequired to relocate the object under inspection from the highthroughput X-ray system into a secondary scanning region. Baggage andcargo items, such as checked baggage in an airport environment, enterthe system 2500 from the left 2501. Baggage and cargo items are screenedusing a high-speed X-ray scanner 2503 comprising an electronicallyswitched X-ray source and associated X-ray detection, imagereconstruction and threat detection sub-systems, as disclosed above.

Baggage follows a conveyor system 2511 to a sorting device 2505 which isconfigured as a loop on which baggage and cargo items remain until theyhave been security cleared for onwards travel, at which point thesorting system 2505 may eject the baggage or cargo item to one of anumber of destinations 2507. In an airport, each destination willpreferably correspond to a direction of a specific departing flight.

Baggage and cargo items that are marked as a threat following X-rayscanning, automatic detection and human visualisation are routed by theconveyor system and sorting device 2511, 2505 to a second confirmatorysensor 2509. Advantageously in this design, the confirmatory sensor 2509is allowed to take a considerable period of time to analyze the baggageor cargo item without impeding the flow of baggage and cargo itemsthrough to their destination.

In some situations, it is advantageous that only cleared bags areallowed into the main sorting system. Referring to FIG. 26, a secondaryloop 2617 is provided for the confirming sensor 2609. Here, baggage andcargo items, which enter from the left 2601, that have been cleared bythe X-ray system 2603 proceed straight through a divert point 2615 andonwards to the main sorting loop 2605 and to their ultimate destination2607. Baggage that has been marked as potentially having one or morethreat items by the X-ray system 2603 are subsequently viewed by anoperator while the baggage and cargo items continue towards the divertpoint 2615 on the conveyor system 2611. If the operator has not cleareda baggage or cargo item by this point 2615, the item is automaticallydiverted into a side loop 2617 which contains the confirmatory sensor2609. Baggage or cargo items which still require additional manualscreening can be diverted to a screening area 2613 while cleared baggagecan be sent onward to the main sorting loop 2605.

In both of the aforementioned embodiments, the conveyor system isadvantageously formed from multiple short conveyor sections (typically1.5 to 2 m long) such that a plurality of items can be queued forscanning. Preferably, queuing slots for 5 to 20 baggage and cargo itemsshall be made available. Baggage and cargo items are passed one item ata time into the confirmatory sensor. If the human operator has completedtheir inspection of a baggage or cargo item while the item has beenqueuing and has marked the item as being clear, then the baggage orcargo item can be passed straight through the confirmatory scannerwithout further delay and back into the main sorting loop, 2505, 2605.Those items which still remain classified as threat items are thensubject to investigation by the confirmatory sensor, 2509, 2609.

If the confirmatory sensor, 2509, 2609 clears an item, it is returnedback to the main sorting loop, 2505, 2605 and continues its journey toits ultimate destination. In the event that the confirmatory sensor,2509, 2609 confirms a threat baggage or cargo item, the item is passedthrough a divert point 2619 to a holding room 2613 where the baggage orcargo item can be reconciled with the passenger or owner of the item andsubsequently hand searched by the appropriate authorities.

X-ray systems detect explosive devices and materials with a high degreeof confidence, but generally cause a false alarm rate on typicallybetween 10% and 30% of all the baggage and cargo items that have beeninspected. These reject items are viewed by one or more operators whotypically are able to resolve between 90% and 99% of all the threatsidentified by the automatic explosives detection algorithms in the X-raysystem. Therefore, the remaining items are those that need to be scannedby the confirmatory sensor.

Accordingly, for a baggage or cargo line with 1800 items per hour at theinput, in one embodiment, up to 600 items per hour may be identified aspotential threat items by the automatic explosives detection algorithmsof the X-ray system and designated for visual inspection. Of these, upto 60 items per hour may be marked as being a potential threat by theinspectors. In another embodiment, as few as 180 items per hour may beidentified as potential threat items by the X-ray system automaticexplosives detection algorithms and designated for visual inspection. Ofthese, up to 2 items per hour may be marked as being a potential threatby the inspectors following visual inspection.

Therefore, the secondary, confirmatory sensor should be designed to takeonly a few minutes to complete its analysis and confirm the nature ofthe materials identified in the baggage or cargo item. This will allowunimpeded flow of baggage and cargo items through the system with nomore than a few minutes delay for any baggage or cargo item for thepurposes of high integrity security screening. Accordingly, the presentinvention employs high throughput embodiments of the secondaryconfirmatory system.

Confirmatory Sensor Embodiment One Nuclear Quadrupole Resonance (NQR)

In one embodiment, the confirmatory sensor comprises a system forconducting a nuclear quadrupole resonance (NQR) measurement. Here, it isknown that certain nuclei, in particular nitrogen and chlorine, possessa significant magnetic quadruple moment. Normally, the magneticquadrupole moments of the individual spinning nuclei in a sample ofmaterial are aligned in random orientations. Under the application of astrong applied magnetic field, the individual magnetic quadrupolemoments of the nuclei in the material under inspection line up with theapplied magnetic field, thereby forming a weak magnetic field acting inthe opposite orientation to the applied field. An applied field may bein the range of 10 to 100 milli Tesla while the generated field due tothe aligned nuclei may be only in the fempto Tesla range. Once theapplied magnetic field is switched off, the magnetic dipoles begin tomove out of alignment and the magnitude of the combined magnetic fieldstarts to the reduce.

The strength of the magnetic field in the first place depends on thetype of nuclei and the concentration of these nuclei in the materialunder investigation. The rate at which the field due to the nucleibuilds up under the influence of the applied magnetic field and how itdissipates again once the applied magnetic field is removed is dependenton the local chemical environment and lattice structure of the nucleiwithin the material under investigation.

Due to the small size of the magnetic field that is generated due toalignment of nuclei in the sample of interest by the applied magneticfield, measurement of signal due to the generated field is generallynoisy. Therefore, in order to build up the signal-to-noise ratio in themeasurement it is advantageous to repeat the measurement many times andto process the signals following each applied field stimulus into onecollective signal.

Referring to FIG. 27, a stimulating coil and associated electronics fora NQR confirmatory sensor 2700 is shown. Here, the coil 2702 is shown asa single turn which extends to a suitable distance such that the item tobe inspected can be contained within the three dimensional envelope ofthe coil, defined as the internal region 2703. Preferably, the coil 2702is fabricated from a material with low resistivity to simplify tuningthe resonant circuit of the coil and to reduce electrical powerdissipation in the coil 2702 which can result in unwanted thermalheating. A suitable coil material is copper.

A signal generator 2708, preferably a digitally programmable signalgenerator, is used to drive the coil 2702 via a power amplifier 2710with suitable bandwidth, typically being up to 10 MHz. Preferably, thepower amplifier 2710 drives the coil 2702 through a fast acting powerswitch 2705. This power switch 2705 provides isolation between the highcurrent applied magnetic field and the sensitive amplifier 2704 which isused to detect the resulting magnetic signal from the material underinspection.

A high gain, high sensitivity, amplifier 2704 is connected via thereactive coupling components 2705 to the coil 2705 used to stimulate thematerial. This amplifier 2704 is typically designed to reject commonmode signals and those ambient signals which are not in the frequencyrange of interest from its analog output signal. A following signalprocessor unit 2706 digitises the analog signal from the high gainamplifier 2704 and applies suitable digital filtering, such as fittingexponentially decaying functions to the envelope of the detected signal.The system determines the relaxation times of the induced magneticquadrupole signals. These relaxation times are dependent on both thenucleus itself and on the chemical environment and lattice structure inwhich the nucleus is situated.

The applied magnetic field is typically pulsed with a predeterminedpulse sequence to maximise the induced signal and to provide timebetween pulses for the induced signal to be recorded and the signalsprocessed as appropriate. FIGS. 28a and 28b show an exemplary pulsesequence for the applied signal and the output, respectively. Referringto FIG. 28a , The applied signal is typically asserted on the order of400 microseconds. This provides time for the signal to build close toits saturation level. Once the applied stimulus is removed, a short deadtime follows (typically 50 to 150 microseconds) for residual eddycurrents in the coil to disappear prior to signal acquisition, as shownin FIG. 28b . The output signal is usually timed over a 500 to 1000microsecond period. The period of the pulse sequence is thus typicallyin the range 200 Hz to 2 kHz. In order to provide a good level ofexplosives detection, a total measurement time of between 1 and 5seconds is usually employed.

The pulse sequence and associated digital filtering is designedspecifically for each type of explosive material that may be ofinterest. Therefore, if a set of four or five compounds is to besearched, the total measurement time may extend out to 30 seconds.

Since in this combined system, the X-ray data will already providea-priori estimation of the type of explosive material that may bepresent, the nuclear quadrupole measurement is targeted first to theanticipated explosive material, and if no match is found, relatedcompounds can be screened. This helps to minimise the examination timewhich is advantageous.

The baggage or cargo item to be inspected is advantageously passed intothe scanning area through a conductive tunnel whose dimension issubstantially smaller than the coil size in order to provide goodimmunity from ambient electromagnetic noise sources.

Confirmatory Sensor Embodiment Two X-Ray Diffraction

In another embodiment, the confirmatory sensor comprises an X-raydiffraction system. X-rays in the energy range of 10 keV to 200 keVpossess an associated wavelength which is commensurate with that of thelattice spacing in known materials. The wavelength of a 10 keV X-ray is1.24×10⁻¹⁰ m (1.24 Angstom) while that of a 200 keV X-ray is 6×10⁻¹² m(0.06 Angstrom). In the situation where the wavelength of a wave and thespacing of scattering objects through which the wave is propagating issimilar to the wavelength, then diffraction of the wave will occuraccording to the Bragg scattering condition

nλ=2d sin θ

where n=order of the diffraction pattern, λ=wavelength of the wave,d=lattice spacing and θ=diffraction angle.

In the case of X-rays, Bragg scattering may be used to determine latticespacing and hence confirm the material type. In a practical X-raysystem, the X-ray source produces not just one energy but a plurality ofenergies, typically in the energy range from 10 keV up to the maximumaccelerating voltage placed on the tube and generally up to 200 keV.These energies are dispersed over the whole energy range, some energiesbeing more likely than others.

FIG. 29 provides an example X-ray spectrum from a standard X-ray tube.Here, the maximum X-ray energy 2905 is defined by the acceleratingvoltage that is applied to the X-ray tube. If the tube is operated withan accelerating voltage of 160 kV, then the maximum possible X-rayenergy is 160 keV. The most likely X-ray energy is of course much lowerthan this. The minimum X-ray energy 2910 is theoretically close to zero,but in reality the minimum X-ray energy is defined by the material typeand thickness of the vacuum support window through which the X-ray beampropagates from the X-ray target to the object under inspection.

If the material under inspection comprises just a single latticeparameter then it may be seen that different diffraction spectra will begenerated for all components of the X-ray spectrum. The net effect is ofa broad diffraction peak compared to the case where a mono-energeticX-ray source is used. Similarly, a real material is generallypolycrystalline or even amorphous in which case further broadening ofthe diffraction spectra will occur. Nonetheless, empirical measurementof diffraction spectra intensity and energy can provide very goodmaterials characterization for even similar materials. Indicative X-raydiffraction spectra for an amorphous material like water and apolycrystalline material such as an explosive are provided for referencein FIGS. 30a and 30b , respectively.

An exemplary embodiment of an X-ray diffraction system for use as aconfirmation sensor with a high speed electronically scanned X-raytomography imaging system is provided in FIG. 31. Here, an X-ray beamemitted from an X-ray tube 3105 is collimated via collimators 3110 intoa beam of rectangular or circular cross section, typically in the range1 mm to 50 mm diameter. The smaller the beam, the more accurate themeasurement is likely to be but the longer that the scan will take tocomplete. A set of detectors 3130 are located opposite to the source3105 but on the opposite side of the object to be inspected 3120. Eachdetector 3130 is located behind secondary collimators 3125, thesecondary collimators 3125 being designed to shield scattered radiationfrom all parts of the object under inspection 3120 except for a smallvolume where the collimated beam intersects with the primary beamvolume. This intersecting region constitutes an inspection volume 3115for that particular detector. Each detector 3130 has a specificsecondary collimator 3125 design such that it interrogates a particularinspection volume 3115 along the length of the primary beam volume.Preferably, each collimated detector element 3130 is configured tosample a different part of the primary beam volume. In this way,parallel data acquisition can be achieved at all intersecting volumesbetween the object under inspection 3120 and the primary X-ray beam.

The secondary collimators 3125 and X-ray detectors 3130 are arranged ina linear configuration such that they each subtend substantially thesame scattering angle to the axis of the primary beam. A suitablescattering angle is in the range of 3 degrees to 10 degrees, an optimalangle being typically 6 degrees.

As shown in FIG. 32, the length of the inspection region 3220 describedby the intersection of the primary beam 3210 and the secondarycollimator beam 3230 is typically in the range from 10 mm to 100 mm andmay advantageously be set at 50 mm as a compromise between detectionefficiency, scanning time and system cost.

The X-ray detectors advantageously provide a suitable energy resolutionsuch that each detected X-ray can be assigned to a particular region ofthe measured X-ray spectrum at each detection point. A suitable detectoris an inorganic scintillation crystal such as NaI(Tl) of CsI coupled toa suitable readout circuit with pulse shaping capability. An exemplaryreadout circuit is shown in FIG. 33 in which a photomultiplier tube 3301converts the optical signal from the photomultiplier into an electronicsignal. Capacitor Cf 3302 is selected to give a suitable gain, whileresistance Rf 3303 is selected to give a suitable pulse duration,typically in the range 50 microseconds to 1000 microseconds. The outputfrom this first stage amplifier 3304 is then passed through a pulseshaping network 3305 comprising a CR-RC filter with time constanttypically in the range 0.1 microseconds to 2 microseconds, the precisevalue being selected to give acceptable count rate performance whilemaintaining a good noise performance. A 1 microsecond shaping time isgenerally preferred. The output from the circuit shown in FIG. 33 ispassed to an analog-to-digital converter (not shown) and to ahistogramming digital memory which builds up a pulse height spectrum. Atypical scintillation crystal size is in the range 10 mm to 50 mmdiameter with a thickness of up to 10 mm.

An alternate detector with improved energy resolution can be selectedfrom the semiconductor detectors, in particular hyper pure germaniumwhich is preferably operated at the temperature of liquid nitrogen (77K) or CdZnWO4 which is commonly operated at room temperature.Semiconductor detectors tend to be more expensive to use and exhibitsome additional complexity in operation, such as the need to use liquidnitrogen.

Once a spectrum from a particular point in the inspection volume hasbeen accumulated, this information can be compared to a database ofempirically derived reference spectra by, for example, using a leastsquares fit of the measured spectrum normalized to a set of referencespectra. Once a fit is determined that is within a certain thresholdvalue, it may be understood that differences between the two spectralshapes are minimal and therefore it is possible to conclude that anexplosive material is likely present.

In a practical embodiment of this invention, the high speed X-ray systemprovides a high degree of information on the location and shape of theanticipated threat material. The X-ray diffraction system is analyzingsubstantially along a certain line through the object. It is thereforerecognized that the three-dimensional X-ray image data may be used totarget the most appropriate trajectory of the X-ray diffraction systembeam through the object to be inspected.

In one embodiment, the image data from the X-ray tomography system isused to reconstruct a three-dimensional set of surfaces that accuratelydefine the exterior shape of the object to be inspected. Once the objectto be inspected arrives at the X-ray diffraction confirmatory sensor,the bag orientation will not be the same as when the X-ray image datawas collected. Therefore, the X-ray diffraction system is provided witha series of video cameras 3401, as shown in FIG. 34, which together canbe used to reconstruct a three-dimensional outer surface for the objectto be inspected.

Using the three-dimensional image of the object exterior calculated bythe X-ray tomography and that calculated by the video cameras, athree-dimensional matrix may be calculated which describes the relativeorientation of the object between the two detection systems. Given thehigh spatial resolution of the three-dimensional X-ray tomography image,an optimal path for the X-ray diffraction beam through the object to beinspected may be calculated. Since the matrix which describes therelative orientation of the bag in the two inspection systems is known,then given a manoeuvrable diffraction probe, it is possible to set thediffraction beam to pass along the most optimal path through the object.

For example, known diffraction sensors can find it difficult to detectexplosive materials in configurations where they are small in onedimension compared to another. In the present invention, the X-ray beammay be targeted to most optimally interrogate the object based ona-priori information from the three-dimensional X-ray tomography image.

Referring to FIG. 34, a rigid but moveable boom 3402 is provided toallow motion of the X-ray sensor and X-ray tube in two dimensions. Asshown in FIG. 34, the diffraction detection package 3403 is arranged tolie substantially parallel to the direction of motion of the conveyor3404. In one embodiment, the detection package is 200 mm to 1200 mm longand still fits in a compact equipment footprint. This is important foruse of the equipment in space constrained environments like an airportbaggage hall. The X-ray source 3405 and its associated collimators (notshown) are fixed to the same rigid but moveable boom such that therelationship between the detector package 3403, the X-ray source 3405and its collimators remains fixed independent on the position of theboom.

Referring to FIG. 35, the boom is fixed to a control system that allowsthe diffraction beam to be rotated from a substantially verticalorientation to a substantially horizontal orientation. Further, thecontrol system allows the boom to be shifted up and down in the verticaldirection. This control system comprises a lift 3501 for adjusting theheight and a rotating bearing assembly 3502 to provide the desiredmotion of the beam between the X-ray source 3503 and the detectorpackage 3504. One of ordinary skill in the art would appreciate that thelayout provided in the drawing is an example only, and other boom andcontrol system configurations can be adopted to achieve the same result.

Given this beam steering configuration, the optimized beam path iscalculated by interrogating the three-dimensional X-ray tomography imagewith a set of beams that have been transformed from the X-raydiffraction system frame of reference using the matrix determined byvideo analysis of the object outline. Once an optimal beam trajectoryhas been calculated, the X-ray diffraction probe is adjusted into theappropriate position and the object moved using the conveyor until theoptimal scan line has been achieved. At this point the X-ray beam isswitched on and the data collection period starts. After a suitabletime, typically between 1 and 5 seconds, the X-ray diffraction signalsare compared with known reference spectra. If a clear match is observedthe scan can be terminated. If not, the beam is switched back on and themechanical assembly is moved around the anticipated inspection point inthe object under inspection by relatively small distances, typically byless than 100 mm in the vertical direction and up to 10 degrees in therotation plane. The data is evaluated at all times during this processwhich may take up to several minutes. At the end of the inspectionperiod, if no diffraction data is observed which matches a known threat,the scan is terminated and the baggage or cargo item under inspection iscleared for onward transit.

In a further embodiment of this invention, the tunnel surrounding theobject under inspection forms the coil assembly for a nuclear quadrupolemeasurement system and the X-ray diffraction probe analyses the objectthrough the coil assembly. The two sets of data, nuclear quadrupoleresonance and X-ray diffraction, can be acquired simultaneously orsequentially as time permits.

Confirmatory Sensor Embodiment Three X-Ray Backscatter Imaging

In another embodiment, the confirmatory sensor comprises an X-raybackscatter imaging system.

X-ray backscatter is produced when X-rays undergo a Compton interaction.Here, the scattered X-ray is left with less energy than it had beforethe collision, the difference in energy being delivered to an electronin the material under investigation. There is a good probability thatthe X-ray will be scattered back in the direction from which it came andthis backscattered X-ray can be detected with one or more X-ray sensorslocated adjacent to the source of X-rays. The direction of the scatteredX-rays is independent of the direction of the input beam and thereforethere is only weak spatial correlation between the backscattered signalfrom a particular object and the backscatter signal detection signal.

It is therefore preferable to incorporate a collimator into the system,between the X-ray source and the object under investigation, which canproduce a one-dimensional scanning pencil beam of X-rays. As this beamscans, the detector signal is correlated with the current beam position,thereby forming a one-dimensional image of the object underinvestigation. If the object is then scanned past the plane of the X-raybeam, and data from the X-ray detectors is also correlated with the scanrate of the conveyor, then a two-dimensional X-ray backscatter image ofthe object under inspection will be produced. An effective pencil beamwidth will be in the range from 1 mm to 10 mm and will preferably be setat 2 mm in order to provide a good balance between spatial resolutionand signal-to-noise ratio. At a conveyor speed of 0.5 m/s an optimizedsystem will use a chopper wheel with four collimation ports operatingwith a rotation speed of 60 revolutions per second.

X-rays are scattered preferentially by high atomic number materials butare also absorbed preferentially by high atomic number materials.Therefore, the backscatter signal is dominated by signals from lowatomic number materials and can be used to see low atomic numbermaterials close to the surface of the object closest to the X-raysource.

In one embodiment of this invention, referring to FIG. 36, a scanningwheel collimator 3605 provides a pencil beam of X-rays, by collimatingX-ray emissions from an X-ray source 3615, which scans across the topsurface 3655 of the baggage or cargo item being inspected 3645. X-rays3635 which backscatter in the cargo are detected in adjacent X-raydetector blocks 3625. These detector blocks 3625 may be positioned atthe end of the scanning region, or preferably they can be locatedparallel and adjacent to the scan line in an orientation 90 degreesrotated around from that shown in FIG. 36. As the baggage or cargo item3645 is scanned, an image is formed which can be correlated withfeatures that are located in the three-dimensional X-ray image.

It is preferable to match the orientation of the baggage or cargo itemin the backscatter system to its orientation when it passed through theX-ray tomography scanner in order that the two sets of image data may becorrelated. Although such a probe will not automatically confirm thepresence of an explosive material, it may be used to provideconfirmation of relative atomic number against the image generated bythe X-ray tomography unit.

Confirmatory Sensor Embodiment Four Trace Detection

In another embodiment, the confirmatory sensor comprises a tracechemical detector.

Certain explosive materials emit a chemical signature through vapourthat emanates from the material at room temperature. This vapour may bepresent typically at parts per million to parts per billion in the airaround a baggage or cargo item to be inspected. The actual concentrationof signature molecules depends on the vapour pressure of the materialitself as well as the rate of gas exchange between the baggage or cargoitem to be inspected and the surrounding air. For example, ashrink-wrapped baggage item will reduce the exchange of gas between anexplosive material within the baggage and the surrounding air.

In one embodiment, the item to be inspected is passed into a chamberusing a conveyor system. An exemplary chamber 3701 is shown in FIG. 37with a conveyer 3705 passing through. Referring to FIG. 37, the chamber3701 is sealed using a suitable mechanism such as doors 3702, 3703 thatopen and close to seal the entrance and exit ends of the chamberrespectively, or shutters that drop down to close chamber. Attached tothe chamber is a small vacuum pump 3704. The pump 3704 operates suchthat once the doors have been closed, the chamber 3701 can be broughtdown to a soft vacuum (typically in the range 50 to 100 mBar). At thispoint, trapped air and other gasses that have been in the baggage orcargo item are drawn out into the main chamber volume. One or more tracedetection systems then analyze the residual gas in the chamber 3701 forthe presence of explosive chemicals.

After the initial purge of air, the chamber is sealed to ensure vapourfrom the article under inspection remains within the chamber rather thanbeing vented to atmosphere. After a suitable time, typically 10 secondsto 5 minutes, the signal analysis from the trace detection apparatuswill be complete and can be correlated against the present of explosivematerials that were predicted from the three-dimensional tomographicimage system.

If a strong correlation is present, the baggage or cargo item can bemarked as reject and passed to a reconciliation area as required. If anexplosive material is identified that was not predicted by the X-raysystem, the item can again be marked as reject. If no explosive materialis identified by the trace detection system, a decision can be made toeither hand search the baggage or to mark the item as cleared foronwards travel.

In a further aspect of this invention, the use of the trace detectionequipment described here can be combined with the X-ray diffractionsensor system and/or the nuclear quadrupole resonance system. In oneembodiment, up to three independent methods can be used simultaneouslyand in the same equipment for detection of explosive materials anddevices. An algorithm may then be used to determine with what certaintya suspect item of baggage or cargo may be released for onwards travel.For example, each independent sensor can rank a unlawful objectsuspected during the first round of screening as ‘Not Present’,‘Possibly Present’ and ‘Definitely Present’. If one or more ‘DefinitelyPresent’ measurements are obtained, the item should be restricted foronwards travel. If all sensors respond with ‘Not Present’, the item canbe cleared for onwards travel. If two or more sensors respond with‘Possibly Present’, the item should be restricted from onwards travel,and so on. Such a voting system provides a high degree of certainty indeciding the release of suspect baggage and cargo items for onwardtravel.

It shall be clear to those of moderate skill in the art that such amulti-layered approach using identical sensors, but with an expandeddatabase, can be used in exactly the same way for the detection ofcontraband such as narcotics, tobacco products and currency, besidesthreat materials.

The above examples are merely illustrative of the many applications ofthe system of present invention. Although only a few embodiments of thepresent invention have been described herein, it should be understoodthat the present invention might be embodied in many other specificforms without departing from the spirit or scope of the invention.Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention may be modifiedwithin the scope of the appended claims.

We claim:
 1. A method for identifying objects in a container,comprising: performing a first stage screening, comprising: arranging astationary X-ray source to generate X-rays from a plurality of X-raysource positions around a scanning region, arranging a first set ofdetectors to detect X-rays transmitted through the scanning region, andarranging a second set of detectors to detect X-rays scattered withinthe scanning region; processing data output from the first set ofdetectors using at least one processor to generate at least onetomographic image; processing data output from the second set ofdetectors to generate scatter image data; and, performing a second stagescreening comprising at least one of a NQR-based screening process,X-ray diffraction based screening process, X-ray backscatter basedscreening process, or trace detection based screening process.
 2. Themethod of claim 1 wherein the at least one processor outputs dataindicative of a suspect object in the container.
 3. The method of claim1 wherein the at least one processor outputs a signal indicating saidcontainer should be subject to the second stage screening only if thefirst stage screening identifies a suspect object in the container. 4.The method of claim 1 wherein the at least one processor outputs asignal indicating said container should not be subject to the secondstage screening only if the first stage screening does not identify asuspect object in the container.
 5. The method of claim 1 wherein thesecond stage screening outputs a signal indicative of whether a suspectobject exists in the container and wherein said output of the secondstage screening, said tomographic image data, and said scatter imagedata are used to determine if the suspect object is illegal.
 6. Themethod of claim 1 wherein said first stage screening is performed inparallel with said second stage screening.
 7. The method of claim 1wherein said first stage screening is performed serially with respect tosaid second stage screening.
 8. The method of claim 1 wherein the firststage screening is used to analyze at least one of the tomographic imagedata or scatter image data to determine a type of material of an objectin the enclosure.
 9. The method of claim 8 wherein the second stagescreening is used to conduct a nuclear quadrupole measurement based onthe type of material determined by the first screening system.
 10. Themethod of claim 1 wherein the second stage screening is used to conductan X-ray diffraction based screening based on the tomographic imagegenerated by the first stage screening.
 11. The method of claim 1wherein said stationary X-ray source is an electronically scanned X-raysource.
 12. A method for identifying objects in a container, comprising:performing a first stage screening comprising: arranging a stationaryX-ray source to generate X-rays from a plurality of X-ray sourcepositions around a scanning region; arranging a first set of detectorsto detect X-rays transmitted through the scanning region; processingdata output from the first set of detectors to generate at least onetomographic image, using at least one processor; and conveying saidcontainer from a first stage screening area to a second stage screeningarea, wherein the second stage screening comprises at least one of aNQR-based screening process, X-ray diffraction based screening process,X-ray back-scatter based screening process, or trace detection basedscreening process.
 13. The method of claim 12 wherein the at least oneprocessor outputs data indicative of a suspect object in the container.14. The method of claim 12 wherein the at least one processor outputs asignal indicating said container should be subject to the second stagescreening only if the first stage screening identifies a suspect objectin the container.
 15. The system of claim 12 wherein the secondscreening system outputs a signal indicative of whether a suspect objectexists in the container and wherein said output of the second stagescreening and said tomographic image data are used to determine if thesuspect object is a threat.
 16. The system of claim 12 wherein saidfirst stage screening is performed in parallel with said second stagescreening.
 17. The system of claim 12 wherein said first stage screeningis performed serially with respect to said second stage screening. 18.The system of claim 12 wherein the first stage screening is used toanalyze the tomographic image data to determine a type of material of anobject in the enclosure.
 19. The system of claim 18 wherein the secondstage screening is used to conduct a nuclear quadrupole measurementbased on the type of material determined by the first stage screening.20. The system of claim 12 wherein the second stage screening system isused to conduct an X-ray diffraction based screening based on thetomographic image generated by the first stage screening.